CYTOGENETIC AND MOLECULAR LOCALIZATION OF TIPE - A GENE AFFECTING SODIUM-CHANNELS IN DROSOPHILA-MELANOGASTER

Voltage-sensitive sodium channels play a key role in nerve cells where they are responsible for the increase in sodium permeability during t he rising phase of action potentials. In Drosophila melanogaster a sub set of temperature-sensitive paralytic mutations affect sodium channel function. One such mutation is temperature-induced paralysis locus E (tipE), which has been shown by electrophysiology and ligand binding s tudies to reduce sodium channel numbers. Three new gamma-ray-induced t ipE alleles associated with either visible deletions in 64AB or a tran slocation breakpoint within 64B2 provide landmarks for positional clon ing of tipE. Beginning with the flanking cloned gene Ras2 a 140-kb wal k across the translocation breakpoint was completed. Germline transfor mation using a 42-kb cosmid clone and successively smaller subclones l ocalized the tipE gene within a 7.4-kb genomic DNA segment. Although t his chromosome region is rich in transcripts, only three overlapping m RNAs (5.4, 4.4, and 1.7 kb) lie completely within the smallest rescuin g construct. The small sizes of the rescuing construct and transcripts suggest that tipE does not encode a standard sodium channel alpha-sub unit with four homologous repeats. Sequencing these transcripts will e lucidate the role of the tipE gene product in sodium channel functiona l regulation.